α3-Galactosyltransferase

Abstract

UDP-Gal:Galβ1→4GlcNAcβ-R α3-galactosyltransferase (α3GalT) is a glycosyltrans- ferase (GT) that acts late in the process of protein and lipid glycosylation. By the action of the enzyme, a Gal residue is attached to exposed Galβ1→4GlcNAc (lacNAc) termini yielding a Galal→3Galβ1→4GlcNAc product structure that cannot be further elongated. The formation of this uncharged sequence provides an alternative for the common chain termination by sialic acid. Although no sugars can be added to the α-Gal residue, the chain underlying the α3-galactosylated terminus may be further modified by the addition of Fuc in al,3-linkage to certain GlcNAc residues, and by the addition of GlcNAc in a β1,6-linkage to specific Gal residues (reviewed in Van den Eijnden 2000). Molecular cloning of α3GalT (Joziasse et al. 1989) has revealed that this enzyme is a type-II membrane protein showing a domain structure that is typical for Golgi-resident GTs. It shows extensive sequence similarity to three other GTs (blood group A and B transferase; Forssman glycolipid synthase), and can be grouped with these enzymes in one GT family. The enzyme is expressed in most mammals, with the notable exception of man, apes, and Old World monkeys.